Psychologists Decipher Brain’s Clever Autofocus Software

Wired Science (Oct, 10. 2011) - It’s something we all take for granted: our ability to look at an object, near or far, and bring it instantly into focus. The eyes of humans and many animals do this almost instantaneously and with stunning accuracy. Now researchers say they are one step closer to understanding how the brain accomplishes this feat.

Wilson Geisler and Johannes Burge, psychologists at the Center for Perceptual Systems at the University of Texas, Austin, have developed a simple algorithm for quickly and accurately estimating the focus error from a single blurry image-something they say is key to understanding how biological visual systems avoid the repetitive guess-and-check method employed by digital cameras. The discovery may advance our understanding of how nearsightedness develops in humans or help engineers improve digital cameras, the researchers say.

In order to see an object clearly, an accurate estimate of blur is important. Humans and animals instinctively extract key features from a blurry image, use that information to determine their distance from an object, then instantly focus the eye to the precise desired focal length, Geisler explains. “In some animals, that’s the primary way they sense distance,” he says. For example, the chameleon relies on this method to pinpoint the location of a flying insect and snap its tongue to that exact spot. Altering the amount of blur by placing a lens in front of its eye causes the chameleon to misjudge the distance in a predictable way.

But scientists didn’t know how biological visual systems estimate blur so well. Many researchers had thought the brain used a system of guessing and checking to get to the answer, much like the way a camera’s auto-focus system works. Basically, the camera changes the focal distance, measures the contrast in the image it sees, and repeats the process until it has maximized the contrast, Burge says.

“This search procedure is slow, often begins its search in the wrong direction, and relies on the assumption that maximum contrast equals best focus—which is not strictly true,” Burge says.

In an attempt to resolve the question of how humans and animals might use blur to accurately estimate distance, Geisler and Burge used well-known mathematical equations to create a computer simulation of the human visual system. They presented the computer with digital images of natural scenes similar to what a person might see, such as faces, flowers, or scenery, and observed that although the content of these images varied widely, many features of the images—patterns of sharpness and blurriness and relative amounts of detail—remained the same.

The duo then attempted to mimic how the human visual system might be processing these images by adding a set of filters to their model designed to detect these features. When they blurred the images by systematically changing the focus error in the computer simulation and tested the response of the filters, the researchers found that they could predict the exact amount of focus error by the pattern of response they observed in the feature detectors. The researchers say this provides a potential explanation for how the brains of humans and animals can quickly and accurately determine focus error without guessing and checking. Their research appears online this week in the Proceedings of the National Academy of Sciences.

“They’ve provided proof that there is enough information in a static image to determine if an object is too close or too far away,” says Larry Thibos, a professor of optometry and vision researcher at Indiana University, Bloomington. “We’ve known for 50 or 60 years that people are very good at knowing whether or not something is in focus. It’s taken this paper to show us how the visual system might accomplish this feat.”

The researchers also added common visual imperfections to their simulations and found that when it comes to judging focus, flaws are actually a good thing.

“What we discovered is that the imperfections in the eye—things like astigmatism and chromatic aberration—actually help it to focus,” Geisler explains. That may help explain why people who have had their astigmatism corrected through laser eye surgery often have trouble focusing for several weeks afterward, Geisler says.

That sort of understanding may have an impact on medical decisions, Thibos says. “People might be tempted to try and perfect nature,” he says, “when maybe it’s better to be a little bit imperfect.”

Monkeys Use Mind Control

ScienceNOW (Oct, 6. 2011) - By implanting electrodes into both the motor and the sensory areas of the brain, researchers have created a virtual prosthetic hand that monkeys control using only their minds, and that enables them to feel virtual textures.

Neuroscientist Miguel Nicolelis of Duke University in Durham, N.C., whose group has been developing so-called brain-machine interfaces, says that one of the pitfalls in these systems is that “no one’s been able to close the loop” between controlling a limb and feeling a physical touch. So he and a group of researchers decided to create a “brain-machine-brain” interface using a virtual system. The researchers implanted two sets of tiny electrodes into a monkey’s brain: one set in the motor control center, and the other in the part of the somatosensory cortex that processes the sensation of physical touch from the left hand. Using the first set, the monkey could control a virtual monkey arm on a computer screen and sweep the hand over virtual disks with different “textures.” Meanwhile, the second set of electrodes fed a series of electrical pulses into the touch center of its brain. A low frequency of pulses indicated a rough texture, whereas high frequency indicated a fine texture (see video), and the monkeys quickly learned to tell the difference.

By giving the monkey rewards when it identified the right texture, the researchers discovered that it took as few as four training sessions for the animal to consistently distinguish the textures from one another, even when the researchers switched the order of the visually identical disks on the screen. The researchers then implanted the electrodes into the sensory region that receives tactile sensations from the foot in a different monkey; this monkey, too, acted as if the virtual appendage (in this case, the foot) was its own, moving it to correctly identify the textures, the team reports online today in Nature.

Although the monkeys are all adults, the motor and sensory regions of their brains are amazingly plastic, Nicolelis says: the combination of seeing an appendage that they control and feeling a physical touch tricks them into thinking that the virtual appendage is their own “within minutes.” And throughout this experiment, the monkey’s own general sense of touch didn’t seem to be affected. “The brain,” Nicolelis says, “is creating a sixth sense.”

“It’s definitely a milestone in brain-computer interfaces,” says neuroscientist Sliman Bensmaia of the University of Chicago, who is developing touch-feedback systems for human prosthetics. Too many of the robotic arms now being developed, even very advanced ones, he says, ignore the importance of touch. “Sensory feedback is critical to doing anything,” he says. Even mundane tasks like picking up a cup require a great deal of concentration so the wearer does not drop or crush it.

The new work is still an early step, however, he says. A biological arm receives countless inputs not only from texture but also from temperature and its position in space.

Nicolelis says his group is currently working on fine-tuning the sensory feedback as well as exploring ways to link the brain and computer wirelessly. After many years of working on brain-computer interfaces, he says, “We’re getting very close to where they may be clinically useful” for paralyzed patients, not just in the lab, and for doctors as well. Touch feedback may allow surgeons, for instance, to perform microscopic surgery or countless other applications. “The brain,” Nicolelis says, “has evolved capabilities that go way beyond the body.”